Multi-stroke engine operation (e.g., varying the number of strokes in a combustion cycle) is one method that may improve fuel economy while maintaining reserve torque capacity for elevated load conditions. During Multi-stroke operation, selected strokes of each cylinder may not follow a typical four-stroke cycle for some conditions, but rather perform extra compression stokes, for example. Operating an engine in this fashion can extend the torque interval, thereby lowering the average torque available from an engine. However, it can also increase fuel economy, at part load, by increasing the cylinder air amount and thermal efficiency. Thus, multi-stroke operation may increase the air charge in a cylinder and extend the cylinder cycle to achieve equivalent part load torque, compared to four-stroke operation. Consequently, the ability to control the stroke count, e.g., 2-stroke, 4-stroke, 6-stroke, and/or 12-stroke, can enable cylinders on-demand to be used to improve fuel economy while retaining torque capacity.
One method to control intake and exhaust valve operation during engine operation is described in U.S. Pat. No. 5,743,221. This method provides a method of controlling valves in a cylinder as a function of desired engine load requirements. The method controls cylinder torque by up to three control processes, full open and full close valve operation, partially open and full close valve operation, and open or closed valves with deactivated fuel.
However, the inventors herein have recognized that the before-mentioned approach can have several disadvantages. Namely, the valve control process may be determined simply based on desired engine load requirements. This may cause an undesirable valve and cylinder mode selection under certain operating conditions. For example, if a valve exhibits an elevated temperature, certain valve and cylinder operating modes may not be desired because they may increase the cylinder load, thereby, further increasing cylinder and valve temperature. Thus, valve selection based simply on desired engine load may increase valve degradation by increasing valve temperature.
In addition, the method mentions multi-stroke cylinder operation, but provides no method to determine if multi-stroke operation is appropriate. For example, multi-stroke cylinder operation can increase cylinder air charge, which can result in increased pressure in the cylinder. If valve degradation has occurred, a valve may not be able to overcome the higher pressure associated with multi-stroke operation. Therefore, determining cylinder and valve operating modes based simply on desired load may produce undesirable results.